Plastics which can be readily welded have become increasingly important for pipeline construction and are used wherever possible, taking into consideration the compatibility of the pipe material with the medium to be transported and the mechanical strength of the corresponding material. Apart from weldability, which on the building sites permits a material-correct, permanent connection, an important part is also played by the usually excellent corrosion resistance of such weldable thermoplastics. Generally, it is only the relatively low mechanical strength which in many cases prevents the widespread use of such pipes in pipeline networks having an average to low operating pressure. If, in spite of this, a weldable material such as, e.g., the widely used polyethylene is used for pipes, all that can be done is to use relatively large wall thicknesses at moderate operating pressures. The upper limit for the operating pressure is about 16 bar with the present, standard thermoplastic pipe materials. Apart from the increased material requirement and the resulting costs, compared with thin pipes made from mechanically stronger materials, the reduced internal diameter, i.e., the lower usable pipe cross-section is disadvantageous and undesirable.
Over the last few years in particular manufacturers of polyethylene pipe materials (PE pipes) have made considerable efforts to develop and market material types having higher. However, the actual rise in usable operating pressure resulting from this has tended to be moderate. Compared with the much higher strength values of glass fibre-reinforced plastic pipes (GFP pipes) also used in pipeline construction, it would not appear to be the correct path. However, the sole use of GFP pipes also fails to offer the desired, wide range of applications, because such materials are subject to limitations of suitability with resurrect to the media carried in such pipes (e.g. corrosion by chemical attack, swelling by diffusion, etc.) and limitation as to the joining technology, which is complicated, expensive and difficult to handle, particularly on the construction site (e.g. tight adhesive joint). Although adhesion would constitute the material-correct joining procedure, it is associated with handling problems and quality risks, particularly in pipelines laid in the ground, and particularly because the tightness of the joint, which has absolute precedence, is mainly dependent on the skill of the pipe layer.
Thus, problems and limitations must be accepted when using GFP pipes and limitations must also be accepted when using thermoplastic, e.g., PE pipes. Each pipeline system has its own prior art and can be used in a safe and reliable manner in its own restricted field. PE pipes can be joined in a tight permanent manner because of the simple and now standardized electrical welding, which is the material-correct joining procedure, and PE pipe materials also have neutral behavior with respect to very many fluids carried. However, care must be taken at higher operating pressures. GFP pipe systems are much more difficult to join to form tight joints and/or are subject to a considerable long-term serviceability risk with respect to such pipe junctions. GFP material pipe systems have a much higher resistance to operating pressures, but there is a certain reduction in the number of fluids which can be carried.